Pratt: F135 Fan Fix Simple, Cheap
Posted by
Graham Warwick at 9/18/2009 4:49 PM CDT
Pratt & Whitney says it's standard industry practice - clip the tip of a blade to remove the piece that's susceptible to damage. And that's what the manufacturer plans to do with the fan blades on its F135 engine for the F-35, after a piece of the tip of a first-stage fan blade broke off during durability testing.
Pratt says the "potential cause" of the piece breaking off was an aerodynamic disturbance caused by a worn bushing ahead of the fan. The bushing is a cylindrical metal part used to seat, or locate, a component in the fan inlet case. Tear-down of the engine revealed all the bushings were severely degraded and some were missing.
That's not as bad as it might sound, because
the blade damage occurred 2,455 cycles into a 2,600-cycle durability test of the initial service release (ISR) engine for production F-35As. That's the equivalent of eight years of in-service operation, Pratt says. When the tip broke off,
the engine was 5 hours into a supersonic high-cycle fatigue test designed to deliberately excite blade vibration.
Pratt says the "minor modification" to be made immediately to all ISR engines will be to clip the corner off the tip of the fan blade at its trailing edge, removing the piece that broke off and "alleviating the potential" for it to fail. This will not degrade the engine's performance, the company stresses.
Engines for flight-test F135s are not affected, although the bushings are the same, because they have a "first-generation" fan that has already passed the required durability testing.
The ISR engine has a "second generation" fan with lighter integrally bladed rotors. The bushings will be inspected periodically for wear until a new design is developed under the F135 component improvement program.